In a prior installation, shown in FIG. 1, operation is as follows:
With the spectrometer cell 1 turned off, and with the first valve 2, the second valve 3, and the third valve 4 all closed, the mechanical primary pump 5 is switched on, and the second valve 3 is opened.
After a few minutes, pressure gauge 6 indicates a pressure of less than 1 mb, and starts up the turbomolecular pump 7.
Once the turbomolecular pump has reached its nominal speed of rotation, the pressure indicated by pressure gauge 8 falls from 10.sup.-2 mb to 10.sup.-6 mb and the filament of the spectrometer cell 1 may be switched on.
The second valve 3 is closed and the first valve 2 is opened. The primary pump 5 then evacuates the part 9 under test. Once the pressure in the part under test, as measured by a pressure gauge 10, falls below 1 mb, the first valve 2 is closed and the second valve 3 is opened.
After a few minutes, the third valve 4 may be opened slowly in such a manner that the pressure indicated by the pressure gauge 8 does not rise above 10.sup.-4 mb. Leakage is measured while the third valve 4 is fully open.
In such an installation, and using the above operating method, it takes a relatively long time to pump out the part 9.
The aim of the present invention is to reduce the time required to pump out the part under test.